#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>

#define RSA_KEY_SIZE 256 // For a 2048-bit key
#define HASH_SIZE 20    // SHA-1 hash size
#define MGF_MASK_SIZE (RSA_KEY_SIZE - HASH_SIZE - 2)

void mgf1(const uint8_t *seed, size_t seed_len, uint8_t *mask, size_t mask_len) {
    // Simple MGF1 implementation using SHA-1 (in practice use a secure hash function)
    // This function is a placeholder. Real MGF1 implementation should be used.
}

void oaep_encode(const uint8_t *message, size_t message_len, uint8_t *encoded, size_t *encoded_len) {
    uint8_t lhash[HASH_SIZE] = {0}; // Should be a hash of the label, simplified here
    uint8_t db[MESSAGE_LEN + MGF_MASK_SIZE];
    uint8_t seed[HASH_SIZE] = {0};  // Should be random, simplified here

    memset(db, 0xFF, MGF_MASK_SIZE); // Padding string PS
    db[0] = 0x00; // Start of message block
    db[1] = 0x01; // Separator
    memcpy(db + 2, message, message_len); // Message
    mgf1(seed, HASH_SIZE, db, MGF_MASK_SIZE); // Mask Generation Function

    // Output the encoded message
    memcpy(encoded, db, RSA_KEY_SIZE);
    *encoded_len = RSA_KEY_SIZE;
}

void oaep_decode(const uint8_t *encoded, size_t encoded_len, uint8_t *message, size_t *message_len) {
    uint8_t db[RSA_KEY_SIZE];
    uint8_t seed[HASH_SIZE] = {0};  // Should be same as used in encoding

    memcpy(db, encoded, RSA_KEY_SIZE);
    mgf1(seed, HASH_SIZE, db, MGF_MASK_SIZE); // Mask Generation Function

    // Extract message
    if (db[0] != 0x00 || db[1] != 0x01) {
        fprintf(stderr, "Invalid OAEP padding.\n");
        exit(EXIT_FAILURE);
    }
    *message_len = RSA_KEY_SIZE - MGF_MASK_SIZE - 2;
    memcpy(message, db + 2, *message_len);
}

int main() {
    uint8_t message[] = "Hello, World!";
    uint8_t encoded[RSA_KEY_SIZE];
    uint8_t decoded[RSA_KEY_SIZE];
    size_t encoded_len, decoded_len;

    oaep_encode(message, strlen((char *)message), encoded, &encoded_len);
    printf("Encoded data:\n");
    for (size_t i = 0; i < encoded_len; i++) {
        printf("%02X ", encoded[i]);
    }
    printf("\n");

    oaep_decode(encoded, encoded_len, decoded, &decoded_len);
    printf("Decoded message: %.*s\n", (int)decoded_len, decoded);

    return 0;
}
